Honestly I didn't have any real problems with the Voice Acting from what I've played so far. And the cut scenes that are there are pretty good, I just wish there were more of them instead of just codec like scenes so often.I totally agree on cheaping out on the story elements. Better cutscenes and voice acting would have given it more of the AA-AAA game feel but probably wasn't in the budget.
Like every enemy in the game (like, everyone of them), they can hear you and get alerted even if you crouch run up to them. It's just that most of them are slower to turn around than the Mortar. If you crouch walk, you can successfully get behind everything.
I always crouch walk to sneak up on things. It works for everything except Mortar's, they turn around without fail so far.